Cross-point memory and methods for fabrication of same

ABSTRACT

The disclosed technology relates generally to integrated circuit devices, and in particular to cross-point memory arrays and methods for fabricating the same. In one aspect, a method of fabricating cross-point memory arrays comprises forming a memory cell material stack which includes a first active material and a second active material over the first active material, wherein one of the first and second active materials comprises a storage material and the other of the first and second active materials comprises a selector material. The method of fabricating cross-point arrays further comprises patterning the memory cell material stack, which includes etching through at least one of the first and second active materials of the memory cell material stack, forming protective liners on sidewalls of the at least one of the first and second active materials after etching through the one of the first and second active materials, and further etching the memory cell material stack after forming the protective liners on the sidewalls of the one of the first and second active materials.

CROSS REFERENCE

The present Application for Patent is a divisional of U.S. patentapplication Ser. No. 14/189,490 by Ravasio et al., entitled “Cross-PointMemory and Methods for Fabrication of Same,” filed Feb. 25, 2014,assigned to the assignee hereof, and is expressly incorporated byreference in its entirety herein.

REFERENCE TO RELATED APPLICATIONS

This application is related to U.S. patent application Ser. No.14/189,265 by Sciarillo, entitled “Cross-Point Memory and Methods forFabrication of Same,” filed Feb. 25, 2014, now U.S. Pat. No. 9,577,010,and U.S. patent application Ser. No. 14/189,323 by Song et al., entitled“Semiconductor Structures Including Liners Comprising Alucone andRelated Methods,” filed Feb. 25, 2014, now U.S. Pat. No. 9,484,196.

BACKGROUND OF THE INVENTION Field of the Invention

Subject matter disclosed herein generally relates to integrated circuitdevices, and in particular to cross-point memory arrays and methods forfabricating the same.

Description of the Related Art

Generally, a cross-point memory array refers to a memory array havingmemory elements disposed and electrically connected at cross-junctionsbetween a first set of conductive lines (e.g., word lines) and a secondset of conductive lines (e.g., bit lines) overlapping and crossing thefirst set of conductive lines. Some cross-point memory arraysincorporate materials whose resistance can be changed by a signalapplied across the memory elements. Examples of resistance changematerials include phase change materials.

Fabrication of cross-point memory arrays can include patterning thememory elements having multiple elements in a stacked configuration.Patterning the memory elements, which often involve etching at leastportions of a stack of materials to form a stacked structure, forexample a stack of lines, can result in undesirable effects such asdamaging and contaminating sidewalls of the stacked structure beingetched. Thus, there is a need for fabrication methods that can minimizesuch undesirable effects.

BRIEF DESCRIPTION OF THE DRAWINGS

Claimed subject matter is particularly pointed out in the concludingportion of the specification. However, organization and/or method ofoperation, together with certain objects, features, and/or advantagesthereof, may be better understood by reference to the following detaileddescription if read with the accompanying drawings in which:

FIG. 1 is a schematic three-dimensional depiction of a memory cellaccording to some embodiments.

FIGS. 2A-2N are schematic three-dimensional depictions of intermediatestructures of a memory cell at various stages of fabrication accordingto some embodiments.

FIGS. 3A-3C are cross-sectional depictions of structures of memory cellsaccording to some embodiments, where the cross-sections are taken in they-z plane.

FIG. 4 is a cross-sectional depiction of a structure of memory cellsaccording to some embodiments, where the cross-sections are taken in thex-z plane.

FIGS. 5A-5C are cross-sectional depictions of structures of memory cellsaccording to some embodiments, where the cross-sections are taken in they-z plane

Features in the drawings are not necessarily drawn to scale and mayextend in different directions from that illustrated. While various axesand directions are illustrated to facilitate the discussion herein, itwill be appreciated that the features may extend in differentdirections.

DETAILED DESCRIPTION

As noted above, generally, a cross-point memory array refers to a memoryarray having memory cells disposed and electrically connected at crossjunctions between a first set of conductive lines (e.g., word lines) anda second set of conductive lines (e.g., bit lines) overlapping andcrossing the first set of conductive lines. The memory cells can changetheir memory state in response to a signal, which can be an electricalsignal such as a voltage or a current pulse. Some memory cells,sometimes called the resistance change cells, incorporate resistancechange materials whose electrical resistance can be changed by theelectrical signals provided through the first and second conductivelines connected to the resistance change materials. One category of suchresistance change materials is that of phase change materials, such asbut not limited to chalcogenide materials.

The memory cells in a cross-point memory array can include multipleelements arranged in a stacked configuration. For example, a memory cellcan include a storage element and a selector element, and can alsoinclude electrodes connecting the storage and selector elements andfurther connecting the storage and selector elements to conductivelines. Such electrodes may be employed to prevent reaction orinterdiffusion between neighboring functional elements, but may not beemployed depending upon the selected materials. Patterning the memorycell having the multiple elements, which often involves etching at leastportions of a stack of materials to form a stacked structure, forexample a stack of lines, can result in undesirable effects such asdamaging and/or cross-contaminating sidewalls of the stacked structurebeing etched. For example, during etching, sidewalls of one of thestorage or selector elements can become damaged chemically and/orphysically. Cross-contamination can occur from redeposition of one ofthe etched materials and/or etch byproducts onto another element. Forexample, elements from a selector material can contaminate sidewalls ofa storage material, or vice versa. Similar damage and/orcross-contamination can occur among one or more of materials of thestorage or selector elements, electrode materials, conductive linematerials, or any other material that may be etched as part of the stacketch process. In addition, contaminants can be thermally “driven-in” insubsequent processes, thereby causing unintended and undesirable effectson the final device characteristics as well as fabrication yield. Thus,there is a need for fabrication methods that can minimize suchundesirable effects.

A method of fabricating a memory device according to some embodimentscomprises forming a memory cell material stack and patterning the memorycell material stack. Forming the memory cell material stack includesforming, e.g., depositing, a first active material and a second activematerial over the first active material. In some embodiments, one of thefirst and second active materials comprises a storage material and theother of the first and second active materials comprises a selectormaterial. Patterning the memory cell material stack can include etchingthrough a portion of the memory cell material stack, such as through atleast one of the first and second active materials of the memory cellmaterial stack, and forming protective liners on the sidewalls prior tocompletion of the etching of the entire memory cell material stack.After forming the protective liners, the cell material stack can befurther etched to complete the cell stack etch. Thus, the portion of thememory cell stack whose sidewalls are covered with the protective linersare protected during further etching of the cell stack such that etchproducts that may be liberated during further etching of the cell stackare prevented from contaminating the portion whose sidewalls are coveredwith the protective liner. Etching can be dry etching, includingphysical etching, chemical etching, or a combination of the two as in areactive ion etch.

FIG. 1 depicts a memory cell 10 in a cross-point memory array accordingto one embodiment. The memory cell 10 in FIG. 1 is a resistance changememory cell arranged in a stacked configuration between a column line 20extending in a y direction and a row line 22 extending in an xdirection. While a single memory cell 10 interposed between one columnline 20 and one row line 22 is illustrated, it will be understood that amemory array can include additional memory cells formed at a pluralityof crossings between a plurality of column lines 20 and a plurality ofrow lines 22. Column lines 20 can alternately be referred to as bitlines or digit lines, or more generally as access lines, and row lines22 can alternately be referred to as word lines, or also more generallyas access lines, by convention in the industry, although thesedesignations can also be reversed. The row and column lines 22 and 20are conductive lines configured to carry electrical signals such as, forexample, a voltage or a current pulse, between cells in an array anddriver circuitry. The memory cell 10 includes a memory cell stack 30including a selector element 38 and a storage element 34, and in theillustrated embodiment these elements are separated by a middleelectrode 36. The memory cell stack 30 additionally includes a firstelectrode 40 between the selector element 38 and the row line 22 and asecond electrode 32 between the column line 20 and the storage element34.

Embodiments modified from the illustrated embodiments of FIG. 1 arepossible. For example, while the illustrated embodiment in FIG. 1 showsthe first electrode 40 configured as a line structure laterally confinedin one dimension and extending in the x-direction above the column line22 and the second electrode 32 configured as a structure laterallyconfined in two dimensions, the opposite configuration is possible,where the first electrode 40 is configured as a structure laterallyconfined in two dimensions and the second electrode 32 is configured asa line structure laterally confined in one dimension and extending inthe y-direction below the row line 20. In other embodiments, both thefirst and second electrodes 40 and 32 can be laterally confined in oneor two dimensions. In addition, in other embodiments, the positions ofthe storage element 34 and the selector element 38 within a stackconfiguration may be interchanged with one another. In yet otherembodiments, the selector element 38 may be omitted. In yet otherembodiments, any one of the first, second, and middle electrodes may beomitted, depending upon compatibility or reactivity of adjacent storage,selector and conductive line materials. Additionally, the “row” and“column” designations are interchangeable, and the rows and columns aregenerally perpendicular but may cross one another at other than 90°.

Examples of the storage element 34 include a chalcogenide-based phasechange storage element, a resistive random access memory (RRAM) storageelement (e.g., NiO, Hf0₂, ZrO₂, Cu₂O, TaO₂, Ta₂O₅, TiO₂, SiO₂, Al₂O₃), aconductive bridge random access memory (CBRAM) storage element (e.g.,metal-doped chalcogenide), and/or a spin transfer torque random accessmemory (STT-RAM) storage element, among other types of storage elements.

Examples of the selector element 38 include a two terminal device (e.g.,a switch), such as a diode, an ovonic threshold switch (OTS), a tunneljunction, or a mixed ionic electronic conduction switch (MIEC), amongother two terminal devices. Alternatively, examples of the selectorelement 38 include a three terminal device (e.g., a switch), such as afield effect transistor (FET) or a bipolar junction transistor (BJT),among other switching elements.

In some embodiments, one or both of the storage and selector elements 34and 38 can comprise chalcogenide materials. When both storage andselector elements 34 and 38 comprise chalcogenide materials, the storageelement 34 can comprise a chalcogenide material that can undergo a phasechange that is nonvolatile at room temperature. On the other hand, theselector element 38 can comprise a chalcogenide material that does notundergo a similar nonvolatile phase change.

In some embodiments, the storage element 34 includes a phase changematerial that includes chalcogenide compositions such as an alloyincluding at least two of the elements within theindium(In)-antimony(Sb)-tellurium(Te) (IST) alloy system, e.g.,In₂Sb₂Te₅, In₁Sb₂Te₄, In₁Sb₄Te₇, etc., or an alloy including at leasttwo of the elements within the germanium(Ge)-antimony(Sb)-tellurium(Te)(GST) alloy system, e.g., Ge₈Sb₅Te₈, Ge₂Sb₂Te₅, Ge₁Sb₂Te₄, Ge₁Sb₄Te₇,Ge₄Sb₄Te₇, etc., among other chalcogenide alloy systems. The hyphenatedchemical composition notation, as used herein, indicates the elementsincluded in a particular mixture or compound, and is intended torepresent all stoichiometries involving the indicated elements. Otherchalcogenide alloy systems that can be used in phase change storageelements include Ge—Te, In—Se, Sb—Te, Ga—Sb, In—Sb, As—Te, Al—Te,In—Ge—Te, Ge—Sb—Te, Te—Ge—As, In—Sb—Te, Te—Sn—Se, Ge—Se—Ga, Bi—Se—Sb,Ga—Se—Te, Sn—Sb—Te, In—Sb—Ge, Te—Ge—Sb—S, Te—Ge—Sn—O, Te—Ge—Sn—Au,Pd—Te—Ge—Sn, In—Se—Ti—Co, Ge—Sb—Te—Pd, Ge—Sb—Te—Co, Sb—Te—Bi—Se,Ag—In—Sb—Te, Ge—Sb—Se—Te, Ge—Sn—Sb—Te, Ge—Te—Sn—Ni, Ge—Te—Sn—Pd, andGe—Te—Sn—Pt, for example.

In some embodiments, the selector element 38 includes a chalcogenidematerial electrically coupled to the storage element 34 through themiddle electrode 36 on one side and electrically connected to the rowline 22 through the first electrode 40 on the other side. A selectordevice having a chalcogenide material can sometimes be referred to as anOvonic Threshold Switch (OTS). An OTS may include a chalcogenidecomposition including any one of the chalcogenide alloy systemsdescribed above for the storage element 34. In addition, the selectorelement may further comprise an element to suppress crystallization,such as arsenic (As). Examples of OTS materials include Te—As—Ge—Si,Ge—Te—Pb, Ge—Se—Te, Al—As—Te, Se—As—Ge—Si, Se—As—Ge—C, Se—Te—Ge—Si,Ge—Sb—Te—Se, Ge—Bi—Te—Se, Ge—As—Sb—Se, Ge—As—Bi—Te, and Ge—As—Bi—Se,among others.

The electrodes 32, 36 and 40 can comprise materials that electricallyconnect the operational elements of the memory cell but preventreactions among the materials. For example, where the storage element 34and the selector element 38 comprise chalcogenide materials, it may beadvantageous to place non-reactive conductors between these elements toprevent interdiffusion of their materials, and also between theseelements and their respective neighboring conductive lines, particularlyconductive lines formed of metallic material. Examples of suitableelectrode materials include one or more conductive and semiconductivematerials such as, for example, carbon (C); n-doped polysilicon andp-doped polysilicon; metals including, Al, Cu, Ni, Cr, Co, Ru, Rh, Pd,Ag, Pt, Au, Ir, Ta, and W; conductive metal nitrides including TiN, TaN,WN, and TaCN; conductive metal silicides including tantalum silicides,tungsten silicides, nickel silicides, cobalt silicides and titaniumsilicides; and conductive metal oxides including RuO₂.

In addition, row lines 22 and column lines 20 can comprise anyconductive and/or semiconductive material suitable for formingconductive lines to carry electrical current for accessing the memorycells within the memory array. Examples of conductive/semiconductivematerials suitable for forming row lines 22 and column lines 20 includen-doped or p-doped polysilicon; metals including Al, Cu, and W;conductive metal nitrides including TiN, TaN, and TaCN; and otherconductive materials. In embodiments where one of electrodes 32 or 40 isomitted, the row line 22 or column line 20 can serve as cell electrodesin addition to carrying electrical signals.

In the following, it will be understood that while some embodiments ofmemory cells may be described as having certain features pertaining tostorage elements and selector elements that include chalcogenidematerials, the embodiments are not limited to such storage and selectormaterials. For example, in some embodiments, a memory cell can include astorage element including a chalcogenide phase change material, whileincluding a selector element that does not include a chalcogenidematerial, such as a bipolar junction transistor or a diode. In someother embodiments, another material with similar electrical behavior asa chalcogenide phase change material may be used. In some otherembodiments, a memory cell can include a selector element including anOvonic threshold switching material, while including a storage elementthat does not include a chalcogenide material, such an oxide-basedresistance change oxide material. In yet other embodiments, a memorycell can include a chalcogenide phase change material having bothstorage and selector functionalities without separate selector elementsfor these functions.

FIGS. 2A-2N are schematic three-dimensional depictions of intermediatememory array structures illustrating structures and methods offabricating a memory device according to various embodiments. In anillustrated embodiment, fabrication includes partially etching a memorycell material stack and protecting the exposed sidewalls of thepartially etched memory cell material stack prior to completing the etchto form stacked memory cell structures, such memory cell line stacks.The etch can be part of a patterning process in which a conductive lineis commonly patterned with a memory cell stack. In the illustratedprocess, a lower row line and an upper column line are patterned usingtwo mask patterns, and an intervening memory cell stack is partiallypatterned by each of the two mask patterns.

While not shown for clarity and ease of illustration, it will beunderstood that the illustrated array structures are formed over asubstrate, which can include, among other things, various peripheral andsupporting circuitry, for instance CMOS transistors that form a part ofcolumn and row driver circuitry and sense amplifier circuitry, as wellas sockets and wiring that connect such circuitry to the memory arraythrough the column lines and row lines described above. In addition, thesubstrate may include one or more memory arrays, or “decks” of arrays.As used herein, the term substrate can includes a bulk semiconductorsubstrate as well as integrated structures formed thereover.

As used herein and throughout the specification, “subtractivepatterning” refers to a process sequence where structures to be definedare patterned by the removal of material. For example, a “subtractivepatterning process” may include first lithographically providing etchmask structures overlapping areas to be patterned, followed by etching,such that materials in areas masked by the mask structures are protectedwhile materials in exposed areas are removed by the etch removalprocess. The set of process steps for subtractive-patterning a stack oflayers, can include, for example, providing an etch mask pattern thatcan comprise at one or more of a resist, a hard mask and anti-reflectivecoating. The resist may be patterned by a photolithography process, andthat pattern can be transferred into lower hard mask and/orantireflective layers. Other lithographic techniques are also possible,including processes without hard mask layers. If hard mask layer(s) areincluded, the resist can be removed prior to using hard mask during etchof underlying materials. Thus the etch mask pattern can be provided byresist and/or hard mask layers at the time of transferring the patterninto the materials of interest. The etch mask pattern blocks areascovered by the mask pattern to protect the underlying material frombeing etched (e.g., wet or dry), while the etch mask pattern exposesareas not covered by the mask pattern to etch the exposed region of thematerial(s) to be etched.

Referring to intermediate array structure 100 a of FIG. 2A, according tosome embodiments, a method of fabricating a memory device includesforming a lower conductive material 22 a over a substrate (not shown),forming a memory cell material stack 30 a over the lower conductivematerial 22 a, and forming a first hard mask material 42 a over the cellmaterial stack 30 a. Forming the memory cell material stack 30 a in turnincludes forming a lower electrode material 40 a on the lower conductivematerial 22 a, forming a selector element material 38 a on the lowerelectrode material 40 a, forming a middle electrode material 36 a on thestorage element material 38 a, forming a storage element material 34 aon the middle electrode material 36 a, forming an upper electrodematerial 32 a on the storage element material 34 a, and forming a firsthard mask material 42 a on the upper electrode material 32 a. Each ofthe materials can be formed by blanket depositing a layer on thesubstrate, such as by sputtering or chemical vapor deposition. It willbe understood that memory cell stacks may omit or add materials in otherembodiments.

Referring to intermediate array structure 100 b of FIG. 2B, according tosome embodiments, the method of fabricating the memory device includessubtractive patterning the memory cell material stack of theintermediate array structure 100 a of FIG. 2A to form a partially etchedlower line stack 44 a extending in the x-direction. After a mask ispatterned over it, the substrate with the intermediate array structure100 a can be loaded into a dry etch tool. In one embodiment, theprocesses described below with respect to FIGS. 2B-2E can be conductedin the dry etch tool.

Subtractive patterning to form the partially etched lower line stack 44a includes forming an etch mask line pattern (e.g., photoresist pattern)comprising lines extending in the x-direction and etching the exposedregions between the etch mask pattern lines. At least the upper activematerial is etched. In the illustrated embodiment of FIG. 2B, etchingthe hard mask material 42 a, the upper electrode material 32 a, and thestorage element material 34 a, forms the partially etched lower linestack 44 a including a storage element line 34 b on the middle electrodematerial 36 a, an upper electrode line 32 b on the storage element line34 b, and a hardmask line 42 b on the upper electrode line 32 b. Theresulting intermediate structure 100 b includes partially etched lowerline stacks 44 a separated by shallow trenches.

It will be appreciated that, while in the illustrated embodiment of FIG.2B, the etch process removes part of the storage element material 34 ato expose its sidewalls, similar etch process can remove part of theselector element material 38 a in embodiments where the verticalpositions of the storage element material 34 a and the selector elementmaterial 48 a are switched according to some embodiments.

In addition, it will be appreciated that, while in the illustratedembodiment of FIG. 2B the etch extends to at least the upper surface ofthe middle electrode material 36 a such that the exposed sidewalls ofthe storage material 34 b and of the upper electrode 32 b cansubsequently be protected using a protective liner, the etch process canstop at other depths depending on which material(s) of the intermediatearray structure 100 a of FIG. 2A is sought to be protected duringsubsequent processing. For example, in some embodiments, the etchprocess can subsequently proceed to remove one or more of the middleelectrode material 36 a, the selector element material 34 a, bottomelectrode material 40 a and the lower electrode material 22 a, as willbe understood from the description below of FIGS. 3A-3C.

Still referring to FIG. 2B, it will be further appreciated that, formingthe partially etched lower line stack 44 a can include at leastpartially etching into the material that underlies the storage materialline 34 b, in this case the middle electrode material 36 a, in order toensure exposure of sidewalls through the full thickness of the storagematerial for subsequent protection, which can result in exposure ofsidewalls through part of the thickness of the middle electrode material36 a.

In general, dry etch processes may utilize chemical reactions byemploying various etchants comprising, for example, at least one of ahalide, such as a fluorine-, chlorine-, bromine- or iodine-containingvapor compound to form a volatile chemical compound with the materialbeing removed. Some other dry etch processes may utilize physical energyby employing acceleration of charged species which may be etchantsthemselves, or a combination of the etchants and other species such asinert ions. Yet some other dry etch processes, such as reactive ionetching, may utilize a combination of both chemical reactions andphysical energy for optimized etching performance.

It will be appreciated that some dry etch processes for patterning thepartially etched lower line stack 44 a of FIG. 2B includes multiplesub-processes to enhance the etch rate and/or profile. However, asub-process tailored for removing a particular material within thevarious materials of the intermediate structure 100 a of FIG. 2A canresult in undesirable consequences. For example, after removingmaterials up to and including the storage element material 34 a, if theetch process continues to further remove the middle electrode material36 a and the selector element material 38 a, atoms, molecules, orclusters of etch byproducts from additional materials removed, such asthe selector element material 38 a, can be released and re-depositedelsewhere, for example on sidewalls of the storage element line 34 b.Materials released from the selector material can contaminate thesidewalls or the bulk material of the storage element line 34 b,resulting in unintended changes in electrical performance of theresulting memory cells, such as a shift in the threshold/switchingvoltage of the memory cells, to name one example. Examples of materialsreleased from the selector material 38 a that can be subsequently beincorporated into the storage element material 34 a to undesirably alterits electrical behavior include Te, As, Ge, Si, Pb, Se, Al, C, Bi, andSb among others. As and Se, in particular, can alter the behavior of achalcogenide storage material, such as GST.

Of course, other cross-contamination can occur by any of the materialsremoved, or byproducts of etching, being redeposited on existingsidewalls any of the materials that have been already etched. Suchcontamination can occur during etching, or during subsequent processes,such as a wet-clean process or a gap fill process.

To avoid such unintended changes in the electrical performance of thememory cells that can result from the dry etch processes, it can beadvantageous to form protective liners on sidewalls of etched layersafter partially etching, for example, to form a partially etched lowerline stack 44 a of FIG. 2B, prior to continuing to etch the stack of theintermediate structure 100 b. Thus, in the descriptions that follow, amethod of forming protective liners on sidewalls of partially etchedlower line stack 44 a such as in FIG. 2B will be described.

Referring to intermediate array structure 100 c of FIG. 2C, according tosome embodiments, the method of fabricating the memory device includesforming a protective liner 46 a on the surfaces of the partially etchedlower line stack 44 a of FIG. 2B, including the sidewalls, to form apartially etched lower line stack 44 b having sidewalls covered with theprotective liner 46 a. The resulting intermediate structure 100 cincludes partially etched lower line stacks 44 a (FIG. 2B) separated byshallow trenches (one shown), and the protective liner 46a formed onsidewall and bottom surfaces of the shallow trenches.

In some embodiments, the protective liner 46 a can include afluorocarbon material. As used herein, a fluorocarbon material includesany material having carbon and fluorine that may or may not have longrange order, and includes polymeric chains and amorphous materials. Suchfluorocarbon materials can be deposited in process chambers configuredfor at least one of deposition and etch processes. Advantageously, thefluorocarbon material can be deposited in-situ in the same dry etchchamber used to form the partially etched lower line stack 44 a of FIG.2B, as well as to perform subsequent etch processes for defining thelower line stack. By depositing the protective liner in-situ, additionalprocessing time, equipment and materials can be reduced. For example,the fluorocarbon material deposited using plasma and fluorocarbonreactant gas species that include C and F, such as CH₂F₂, CF, CF₂, CF₄,C₄F₆, C₄F_(s), COF₃ and CHF₃, to name a few.

In some embodiments, forming the protective liner 46 a includesdepositing a conformal liner material which uniformly and substantiallycovers all exposed surfaces of the intermediate structure 100 b of FIG.2B, including the sidewalls of the partially etched lower line stack 44a and inter-line stack regions between adjacent partially etched lowerline stacks 44 a. At least sidewalls of the upper active material arecovered. In the illustrated embodiment, sidewalls of the hard mask lines42 b, upper electrode lines 32 b, storage element lines 34 b, and anyexposed sidewalls of the middle electrode material 36 a are covered. Asused herein, a conformal material has thicknesses on various portions ofdeposited surfaces that are substantially the same. Thus, whenconformal, the protective liner 46 a has substantially the samethicknesses on top surface and sidewalls of the partially etched lowerline stack 44 b, as well as on the middle electrode material 36 abetween adjacent partially etched lower line stacks 44 b.

It will be appreciated that conformal protective liner 46 a canadvantageously enable a subsequent spacer structure formation (describedfurther below). Some process conditions, such as a lower substrate biasduring deposition and /or higher process chamber pressure compared tothe conditions used for the vertical etch resulting in FIG. 2B canfacilitate formation of a conformal protective liner 46 a. Without beingbound to any theory, such process conditions can retard acceleration ofthe etchant species toward the substrate. For example, if dry etchconditions for etching to define the storage material lines 34 b includelower pressures of about 1-20 mTorr and a high substrate bias of about−200 V to −500 V, the pressures can be increased to about 30-50 mTorrand substrate bias reduced to −50 V to +50 V, particularly 0 V (nosubstrate bias) for greater isotropic processing and more conformaldeposition of the protective liner 46 a.

In other embodiments, the conformal protective liner 46 a can bedeposited ex situ, in which case an inorganic material such as siliconnitride can be employed.

It will be appreciated that the thickness of the protective liner 46 acan be tailored for effective protection from cross-contamination, aswell as controlling the resulting variations in the widths of the layersabove and below the interface between the storage element line 34 b andthe middle electrode layer 36 a in FIG. 2C. In some embodiments, theprotective liners can have a thickness between about 5% and about 25% ofa width of the cell line stack 30 b measured at the same verticalheight, or between about 10% and 20%, for instance about 15%. In someembodiments, the protective liners can have a thickness between about 1nm and about 20 nm, or between about 1 nm and about 10 nm, for instanceabout 5 nm.

Referring to intermediate array structure 100 d of FIG. 2D, the methodof fabricating the memory device additionally includes, according tosome embodiments, anisotropically removing horizontal portions of theprotective liner 46 a of FIG. 2C to expose the upper surfaces of thehard mask lines 42 b and upper surfaces of the middle electrode material36 a between the partially etched lower line stacks 44 c. The remainingprotective liner 46 b covers the sidewalls of the partially etched lowerline stack 44 c, including the sidewalls of the storage lines 34 b. Suchanisotropic removal may be referred to as “spacer etch process” in theindustry, which refers to a directional etch process designed to removematerials predominantly in a vertical direction (z-direction) such thatthe protective liner 46 a of FIG. 2C is substantially or completelyremoved from horizontal surfaces (e.g., surfaces formed in the x-yplane), while the protective liner 46 b remains over vertical surfaces(e.g., surfaces formed in the x-z and y-z planes), and can be referredto as a protective spacer. After anisotropically etching to remove theprotective liner material from the bottom surfaces of the shallowtrenches, the resulting intermediate structure 100 d includes partiallyetched lower line stacks 44 a separated by shallow trenches, and theprotective liners 46 b formed on sidewall surfaces of the shallowtrench.

The spacer etch can be conducted in the same dry etch chamber assubsequent etching described with respect to FIG. 2E below. Inembodiments in which the protective liner 46 a is formed in situ withina dry etch tool, the same tool can be employed for the sequence of FIGS.2B-2E.

Referring to intermediate array structure 100 e of FIG. 2E, according tosome embodiments, once the protective liner 46 a of FIG. 2C isanisotropically etched to expose the upper surfaces of the hard maskline 42 b and the upper surfaces of the middle electrode material 36 abetween the partially etched lower line stacks 44 c, the intermediatearray structure of 100 d of FIG. 2D, including remaining thickness ofthe middle electrode material 36 a that may have been partially etchedin the prior step, is further etched to define fully etched lower linestacks 44 d as shown in FIG. 2E. The fully etched lower line stacks 44 dinclude, starting from the bottom, a lower conductive line 22 on thesubstrate, a cell line stack 30 c on the lower conductive line 22 andany remaining first hard mask line 42 b on the cell line stack 30c. Thecell line stack 30 c includes a lower electrode line 40 on the lowerconductive line 22, a selector element line 38 b on the lower electrodeline 40, a middle electrode line 36 b on the selector element line 38 b,a storage element line 34 b on the middle electrode line 36 b, and anupper electrode line 32 b on the storage element line 34 b. It will beappreciated that under some circumstances, the first hard mask material42 a may be substantially removed near the completion of the subtractivepatterning process that forms the lower line stack 44 d. The protectiveliners 46 b protect sidewalls of the upper or second active material,which in the illustrated embodiment is represented by the storageelement line 34 b, during the further etch. In particular, the storageelement line 34 b, the upper electrode line 32 b and any remaining hardmask line 42 b above the middle electrode line 36 b have sidewallscovered by the protective liners 46 b while the remainder of the stackis wider because it is etched under the shadow of the protective liners46 b to form the fully etched lower line stack 44 d of FIG. 2E. Any etchbyproducts that may be released or resputtered during etching to formstructures below the protective liners 46 b, including the middleelectrode line 36 b, the selector element line 38 b, the lower electrodeline 40 and the lower conductive line 22, may be formed on the sidewallsof the protective liners 46 b while being prevented from directlyredepositing on sidewalls of the structures covered by the protectiveliners 46 b, including the storage element line 34 b and the upperelectrode line 32 b.

Referring FIG. 2F, according to some embodiments, the method offabricating the memory device additionally includes removing theprotective liners 46 b to expose sidewalls of the storage element line34 b, the upper electrode line 32 b and any remaining hard mask line 42b. By doing so, the protective liner 46 b, whose surfaces may havebyproducts from etching layers below the spacer-shaped protective liners46 b redeposited thereon as described above, including atoms, molecules,or clusters of the selector element material (e.g., As, Se), can beremoved without directly exposing the sidewalls of the storage line 34 bto the byproducts. In various embodiments, cleaning to remove theprotective liner 46 b can be performed using any suitable wet and/or drycleaning processes known in the art for cleaning dry-etched surfaces.For example, one such cleaning process may include wet and/or vaporcleaning using a solution including dilute hydrofluoric acid (HF) at aconcentration between about 0% and 10%, or between about 0% and 5%.Another such cleaning process may include wet and/or vapor cleaningusing a solution including carboxylic acid at a concentration betweenabout 0% and about 2%, or between about 0% and about 1%, or betweenabout 0% and about 0.5%. In other embodiments, where an ex situdeposited inorganic spacer is employed, the protective liners may remainin place through subsequent processing and may remain in the finalproduct.

Still referring to FIG. 2F, it will be appreciated that as a result ofremoving the protective liner 46 b, different portions of the fullyetched lower line stack 44 e can have different widths depending onwhether the portion was etched before or after forming the protectiveliners 46 b. In the illustrated embodiment, because the protectiveliners 46 b were formed after etching to form the storage element line34 b, the widths of the portions of the fully etched lower line stack 44e above and below the exposed top surface of the middle electrode line36 b can be different by an amount corresponding to the thickness of theprotective liners 46 b in FIG. 2E. In some embodiments, the differencein widths between the wider and narrower portions of the cell line stackcan be between about 10% and about 50% of the width of the cell linestack 30 b measured immediately above the same vertical level, orbetween about 20% and 40%, for instance about 30%. In some embodiments,the protective liners can have a thickness between about 1 nm and about20 nm, or between about 1 nm and about 10 nm, for instance about 5 nm.

Referring FIG. 2G, according to some embodiments, the method offabricating the memory device additionally includes forming a sealingdielectric 47 a on exposed surfaces of the fully etched lower line stack44 e, after removing the protective liners 46 b as described withrespect to FIG. 2F, to form the intermediate array structure 100 g.Unlike the protective liners 46 b formed in situ in the dry etch tool,the sealing dielectric 47 a can remain permanently on at least somesurfaces of the fully etched lower line stack 44 f. In some embodiments,the sealing dielectric 47 a can include a suitable dielectric to protectat least portions of the lower line stack 44 f from subsequent processessuch as subsequent etching to confine the stack in the y dimension (seeFIGS. 2H-2N), cleaning, gapfill and thermal processes. The sealingdielectric 47 a can include oxide materials, such as silicon oxide(e.g., SiO₂) and aluminum oxide (e.g., Al₂O₃), nitride materials, suchas silicon nitride (e.g., Si₃N₄), or combinations thereof, among others.In general, the sealing dielectric 47 a can be formed by a suitableconformal deposition technique such as chemical vapor deposition (CVD)and atomic layer deposition (ALD), including thermal and plasmadeposition techniques.

Referring to FIG. 2H, according to some embodiments, the method offabricating the memory device additionally includes filling spacesbetween adjacent lower line stacks 44 f of FIG. 2G with an isolationdielectric material to form first isolation dielectric regions 48 a.Suitable dielectric materials to fill the spaces can include, forexample, silicon oxide, which may be deposited by suitable gap-fillingprocesses such as high-density plasma (HDP) processes,spin-on-dielectric (SOD) processes, sub-atmospheric chemical vapordeposition (SACVD) processes, and atomic layer deposition (ALD)processes, among others. Once the inter-line spaces between adjacentlower line stacks 44 g are filled with the dielectric material to formthe isolation dielectric regions 48 a, the intermediate array structurecan be chemical-mechanically polished to remove overlying gap filldielectric and any remaining portions of the hard mask and expose asubstantially planar surface comprising alternating surfaces of theupper electrode line 32 b interposed by isolation dielectric regions 48a and sealing dielectrics 47 b to complete formation of the lower linestack 44 g of the intermediate structure 100 h.

Referring now to intermediate array structure 100i of FIG. 21, accordingto some embodiments, the method of fabricating the memory deviceadditionally includes depositing an upper conductive material 20 a onthe planar surface of the intermediate array structure 100 h. The upperconductive material 20 a can comprise substantially similar materialsand can be formed using substantially similar processes as discussedabove for lower conductive material 22 a of FIG. 2A.

Referring to intermediate array structure 100j of FIG. 2J, according tosome embodiments, the method of fabricating the memory deviceadditionally includes subtractively patterning to form a partiallyetched wall structure 50 a including the upper conductive line 20.Forming the partially etched wall structure 50 a includes, in a similarmanner as described above for patterning the lower line stack 44 a (FIG.2B), forming an etch mask pattern comprising lines (e.g., photoresistlines, not shown for clarity) that cross with the underlying lower linestacks 44 g, in the illustrated embodiment extending orthogonally in they-direction, and etching the exposed regions to form the partiallyetched wall structures 50 a. Each partially etched wall structure 50 aincludes an upper conductive line 20 and further includes forming anupper material stack confined in x- and y-directions, including an upperelectrode 32 and a storage element 34. In the illustrated embodiment ofFIG. 2B, similar to FIG. 2B, etching the wall structure 50 a includesstopping the etch process on or within the middle electrode line 36 b,to form the partially etched wall structure 50 a. As discussed in FIG.2B, stopping the etch at the middle electrode line 36 b as shown in FIG.2J is for illustrative purposes only, and the etch can stop at anysuitable vertical position above and below the exposed surface of themiddle electrode line 36 b as shown in FIG. 2J.

Referring to intermediate array structure 100 k of FIG. 2K, according tosome embodiments, the method of fabricating the memory deviceadditionally includes, in a manner similar to that described withrespect to FIG. 2C for forming the protective liner 46 a, forming asecond protective liner 52 a over the partially etched wall structure 50a of FIG. 2J to form the partially etched wall structure 50 b of FIG.2K. Parameters and conditions for forming the second protective liner 52a can be similar to that described with respect to FIG. 2C for formingthe protective liner 46 a.

Referring to intermediate array structure 1001 of FIG. 2L, according tosome embodiments, the method of fabricating the memory deviceadditionally includes, in a manner similar to that described withrespect FIG. 2D for forming the first protective liners 46 b,anisotropically etching the second protective liner 52 a of FIG. 2K toform second protective liners 52 b having spacer profiles. Parametersand conditions for forming the second protective liners 52 b are similarto that described with respect to FIG. 2D for forming the firstprotective liners 46 b.

Referring to intermediate array structure 100 m of FIG. 2M, according tosome embodiments, the method of fabricating the memory deviceadditionally includes, in a manner similar to that described withrespect to FIG. 2E for further etching to form the fully etched lowerline stack 44 d, further etching the intermediate structure 100 l ofFIG. 2L to form the fully etched wall structure 50 d of FIG. 2M. Exceptfor the difference in materials being etched and stopping on the uppersurfaces of the lower electrode lines 40, the parameters and conditionsfor further etching to form the fully etched wall structure 50 d can besimilar to FIG. 2E. Because the cell line stack 30 c (FIG. 2F) isalready patterned in crossing lines, the fully etched wall structure 50d includes the column line 20 overlying pillars of the memory cell stackthat alternate with isolation material 48. In other words, the etchseparates the cell line stack into pillars.

During the further etch, the second protective liners 52 b protectsidewalls in the z-y plane of the underlying upper or second activematerial, which in the illustrated embodiment is represented by thestorage element 34 (FIG. 2J). The remainder of the stack is widerbecause it is etched under the shadow of the second protective liners 52b to form the fully etched upper line stack 44 d of FIG. 2E

As noted above with respect to FIGS. 2B-2E, in one embodiment thesequence of FIGS. 2J-2M is conducted in situ within dry etch toolwithout removing the substrate until after the fully etched wallstructure 50 d is formed. As described above, conditions for verticaldry etching (including high negative substrate bias and relatively lowpressure for anisotropic etching) can be altered to be more conducive toconformal deposition (including lower or no substrate bias andrelatively higher pressures), and an organic second protective liner 52a of FIG. 2K can be plasma deposited as a polymer with the supply offluorocarbon precursors. Conditions can then be returned to thoseconducive to anisotropic dry etching and.

In another embodiment, the substrate can be removed between etchingsteps and an inorganic second protective liner 52 a of FIG. 2K can bedeposited in a separate tool. In this case the resultant secondprotective liners 52 b of FIG. 2M can optionally remain in the finalproduct, in contrast to the process described below with respect to FIG.2N below.

Referring to intermediate array structure 100 n of FIG. 2N, according tosome embodiments, the method of fabricating the memory deviceadditionally includes, in a manner similar to that described withrespect to FIG. 2F for removing the first protective liners 46 b,cleaning to remove the a second protective liners 52 b from thesidewalls of the wall structure 50 d and leave a wall structure 50.Except for the fact that the second protective liners 52 b are removedfrom the sidewalls of the storage element 34, the upper electrode 32 andthe upper conductive line 20, the parameters and conditions for cleaningto remove the second protective liners 52 b can be similar to thatdescribed with respect to FIG. 2F for cleaning to remove the firstprotective liners 46 b.

Similar to FIGS. 2F-2G, any etch byproducts that may be released duringetching to form structures below the second protective liners 52 b inFIG. 2M, including the middle electrode 36 and the selector element 38,can redeposit on the sidewalls of the second protective liners 52 bwhile being prevented from directly redepositing on sidewalls of thestructures covered by the second protective liners 52 b, including theupper or second active material in the form of the storage element 34.In embodiments where the second protective liners 52 b are to remain inthe final product, such etch byproducts can be removed with the removalof the second protective liners 52 b such that the byproducts do notcome into direct contact with the storage element 34 and the upperelectrode 32.

It will be appreciated that, as a result of removing the secondprotective liners 52 b, different portions of the wall structure 50 ccan have different widths at different portions, depending on whetherthe portion is formed before or after the second protective liners 52 ahave been formed, in a manner described with respect to FIG. 2F. In someembodiments, due to the shadowing effect of spacers on both sides, thedifference in widths of the wall structure 50 measured immediately aboveand below the vertical level of the exposed top surface of the middleelectrode 36 can be between about 10% and about 50% of the width of thewall structure 50 measured immediately above the exposed top surface ofthe middle electrode 36, or between about 20% and 40%, for instanceabout 30%.

In addition, although not illustrated, after removing the secondprotective liners 52 b as illustrated in FIG. 2N, the intermediatestructure 100 n can be further processed to form a second sealingdielectric (not shown) on exposed surfaces of intermediate structure 100m, in a similar manner as described in FIG. 2G for forming the sealingdielectric 47 a. In addition, the spaces between adjacent wallstructures 50 lined with the second sealing dielectric can be filledwith a filler dielectric to form second isolation dielectric regions(not shown) and polished (not shown), in similar manner as described inFIG. 2H to expose alternating upper conductive lines 20 (or hard masklines or etch stop materials thereover) interposed by second isolationdielectric regions and second sealing dielectrics.

FIG. 3A illustrates a cross-sectional view of an array structure 200 afabricated according to the illustrated embodiments of FIGS. 2A-2N. Thecross-section in FIG. 3A is taken in the y-z plane corresponding to they-z plane of FIGS. 2A-2N. As described above, in this embodiment, theprotective liners 46 b (not shown) having spacer structures are formedand removed from sidewalls of the partially etched lower line stack 44 c(FIG. 2D) including sidewalls of the storage element 34 and the upperelectrode 32. After removing the protective liners 46 b (not shown) andfurther etching to form the fully etched lower line stack 44 e (FIG.2F), a sealing dielectric 47 is formed to cover the x-z sidewalls of theentire stack below the upper conductive line 20 as shown in FIG. 3A. Asdescribed above, the cross-section of the array structure 200 a shows,as a result of the protective liner 46 b formation, steps form lateralplateau regions on an upper surface of the middle electrode 36 (whichmay be partially through the thickness of the upper electrode material)such that the width of the cell stack immediately below the uppersurface of the middle electrode 36 is greater compared the width of thecell stack immediately above the upper surface of the middle electrode36. As described above, such process flow can be desirable, for example,when materials above the step, such as upper active material in the formof the illustrated storage element 34, are sought to be protected fromthe etching processes of the layers below the step. Other configurationsare possible, as illustrated in FIGS. 3B and 3C, in which similar partsare referenced by like reference numerals.

FIG. 3B illustrates a cross-sectional view of an array structure 300 aaccording to another embodiment. Similar to FIG. 3A, the cross-sectionin FIG. 3A is taken in the y-z plane. In this embodiment, instead ofinitially stopping the etch removal process on the middle electrodematerial 36 a as illustrated in FIG. 2B prior to formation of theprotective liner 46 b, the etch process is stopped on or within thelower electrode material 40 a of FIG. 2B, and the subsequent protectiveliner (not shown) having a spacer structure is formed thereover. Afterremoving the protective liner (not shown) and further etching to formthe fully etched lower line stack, a sealing dielectric 47 is formed tocover the x-z sidewalls of the entire stack below the upper conductiveline 20 as shown in FIG. 3B. As a result of the formation and removal ofthe protective spacer (not shown), steps form lateral plateau regionsare on an upper surface of the lower electrode 40 (which may bepartially through the thickness of the lower electrode material) suchthat the width of the cell stack immediately below the step is greatercompared the width of the cell stack immediately above the step, asillustrated in the resulting cross-section of the array structure 300 ain FIG. 3B. Such process flow can be desirable, for example, whenmaterials above the step are sought to be protected from the etchingprocesses of the lower electrode 40 or the lower conductive line 22, butthe materials of the two active materials (storage element 34 andselector element 38) are compatible.

FIG. 3C illustrates a cross-sectional view of a fully fabricated arraystructure 400 a according to yet another embodiment. Similar to FIGS. 3Aand 3B, the cross-section in FIG. 3C is taken in the y-z plane. In thisembodiment, in addition to stopping the etch removal process once on orwithin the middle electrode material 36 a as in FIG. 3A, to form theprotective liners 46 b, in this embodiment the etch removal processstopped a second time on or within the lower electrode material 40 a,and the protective liners having spacer structures (not shown) areformed thereover twice at both locations. After removing the protectiveliners after forming them at each location (not shown) and furtheretching to form the fully etched lower line stack, a sealing dielectric47 is formed to cover the x-z sidewalls of the entire stack below theupper conductive line 20 as shown in FIG. 3C. The resultingcross-section of the array structure in FIG. 3C has steps forminglateral plateau regions on upper surfaces of the middle and lowerelectrodes 36 and 40 such that a width of the cell stack immediatelybelow the lower step is larger compared to a width of the cell stackimmediately above the lower step, and a width of the cell stackimmediately below the upper step is larger than a width of the cellstack immediately above the upper step. Such process flow can bedesirable, for example, when materials above the upper step are soughtto be protected from etching processes of the middle electrode 36 andthe selector element 38, and in addition, materials above the lower stepare sought to be protected from the etching processes of the lowerelectrode 40 or the lower conductive line 22.

FIG. 4 illustrates a cross-sectional view of an array structure 200 bcorresponding to array structures 200 a, 300 a or 400 a of FIGS. 3A-3C,according to some embodiments. The cross-section in FIG. 4 is taken inthe x-z plane corresponding to the x-z plane of FIGS. 2A-2N. The arraystructure of FIG. 4 can be fabricated using similar processes asdescribed with respect to FIGS. 2A-2N. However, it will be appreciatedthat spacer process used to protect the upper active material duringcolumn line etching, per FIGS. 21-2N and FIG. 4, and the spacerprocess(es) use to protect the upper active material during row lineetching, per FIGS. 2A-2H and 3A-3B, can be employed independently ofeach other or combined. In this dimension also the step can be formed onor within the middle electrode material, as shown.

FIGS. 5A-5C illustrate cross-sectional views similar to those of FIGS.3A 3C, except that the protective spacers 46 have not been removed. Theprotective spacers of the illustrated embodiments can be inorganic tominimize risk of interaction with the active materials (e.g., storageand selector elements) of the memory cell stack. Examples includesilicon nitride and silicon oxynitride. It will be understood that theprotective spacers on the orthogonal walls of the memory cell pillar(the dimension shown in FIG. 4) can similarly be maintained in the finalproduct.

Although this invention has been described in terms of certainembodiments, other embodiments that are apparent to those of ordinaryskill in the art, including embodiments that do not provide all of thefeatures and advantages set forth herein, are also within the scope ofthis invention. Moreover, the various embodiments described above can becombined to provide further embodiments. In addition, certain featuresshown in the context of one embodiment can be incorporated into otherembodiments as well. Accordingly, the scope of the present invention isdefined only by reference to the appended claims.

What is claimed is:
 1. A memory device, comprising: a substrate; a lowerconductive line disposed above the substrate and extending in a firstdirection; and an upper conductive line disposed above the lowerconductive line and extending in a second direction different than thefirst direction; and a memory cell stack interposed between the lowerand upper conductive lines, the memory cell stack comprising an upperactive element and a lower active element, wherein the memory cell stackincludes lateral plateau regions when viewed in the first direction suchthat a first width of the memory cell stack below the plateau regions iswider than a second width of the memory cell stack above the plateauregions.
 2. The memory device of claim 1, wherein the first width of thememory cell stack below the plateau regions is between 10% and 50% widerthan the second width of the memory cell stack above the plateauregions.
 3. The memory device of claim 1, wherein the memory cell stackfurther comprises: an upper electrode between the upper active elementand the upper conductive line; a middle electrode between the upper andlower active elements; and a lower electrode between the lower activeelement and lower conductive line.
 4. The memory device of claim 3,wherein the lateral plateau regions are formed below the upper activeelement.
 5. The memory device of claim 3, wherein the lateral plateauregions are formed below the lower active element and above the lowerconductive line.
 6. The memory device of claim 5, further comprising asecond lateral plateau region formed between the upper active elementand the lower active element.
 7. A method comprising: forming a memorycell stack over a substrate, the memory cell comprising a first activematerial and a second active material over the first active material;the memory cell stack further comprising a lower electrode material, amiddle electrode material, and an upper electrode material, wherein thefirst active material is interposed between the upper electrode materialand the middle electrode material, and wherein the second activematerial is interposed between the lower electrode material and themiddle electrode material; etching the first active material and theupper electrode; forming a first protective liner on sidewalls of thefirst active material, a lateral plateau region, and the upperelectrode.
 8. The method of claim 7, wherein forming the protectiveliner further comprises: forming the lateral plateau region on an uppersurface of the middle electrode.
 9. The method of claim 8, wherein, awidth of the memory cell stack below the upper surface of the middleelectrode is greater than a width of the memory cell stack above theupper surface of the middle electrode.
 10. The method of claim 7,further comprising: prior to forming the first protective liner, etchingthe middle electrode and the second active material; and forming asecond protective liner on the middle electrode and the second activematerial, wherein the lateral plateau region is formed on an uppersurface of the lower electrode.
 11. The method of claim 7, furthercomprising: forming a seal dielectric over the first active material,upper electrode, middle electrode, second active material, and lowerelectrode.
 12. The method of claim 8, wherein: a width of the memorycell stack below the upper surface of the lower electrode is greaterthan the width of the memory cell stack above the upper surface of thelower electrode.
 13. The method of claim 7, wherein forming protectiveliners further comprises: forming spacer structures.
 14. The method ofclaim 7, wherein one of the first active material or the second activematerials comprises a storage material, and wherein the other of thefirst active material or the second active materials comprises aselector material.
 15. A memory device, comprising: a substrate; amemory cell stack over the substrate, the memory cell stack comprising:a first active material; a second active material different from thefirst active material; a first electrode above the first activematerial; a second electrode below the first active material and abovethe second active material; a third electrode below the second activematerial; a first lateral plateau region; a first protective linerformed on the first lateral plateau region; a second lateral plateauregion; and a second protective liner formed on the first lateralplateau region and the second lateral plateau region.
 16. The memorydevice of claim 15, wherein the first protective liner is formed on anupper surface of the first electrode and the second protective liner isformed on an upper surface of the second electrode.
 17. The memorydevice of claim 16, wherein a width of the memory cell stack below thesecond electrode is greater than a width of the memory cell stack abovethe second electrode, and wherein a width of the memory cell stack belowthe third electrode is greater than a width of the memory cell stackabove the third electrode.
 18. The memory device of claim 15, furthercomprising: a sealing dielectric formed over the sidewalls of the firstactive material, upper electrode, middle electrode, second activematerial, and lower electrode.
 19. The memory device of claim 15,wherein one of the first active material and the second active materialis a selector material and the other is a storage material.
 20. Thememory device of claim 15, further comprising: an upper conductivematerial positioned above the first electrode; and a lower conductivematerial positioned below the third electrode.